1. Field of the Invention
The present invention relates to a method of decoding a succession of binary code words y.sub.j (t) received from a transmission channel, which code words result from the transmission over the channel of successive binary code words x.sub.j (t) representative of respective successive values u.sub.j (t) of an analogue signal, where x.sub.j (t)=F(u.sub.j (t)), in which method successive values v.sub.j (t) are generated which are estimated from the succesive received code words y.sub.j (t) to correspond to the successive u.sub.j (t), the estimation process taking the form ##EQU1## where F.sup.-1 denotes the inverse of mapping function F, n is an integer, and each p(x.sub.i .vertline.y.sub.j (t)) is an estimate of the relative conditional probability that x.sub.i was transmitted, y.sub.j (t) having been received.
2. Prior Art
The transmission of analogue signals via digital channels generally requires the conversion of the analogue signal to a sampled digital representative, encoding, transmission over the data link, reception, decoding and reconstruction of the original signal. If error protection is required then it is not unusual for redundant data to be added to enable error correction to to be effected at the receiver. This applies to, for example, pulse code modulation (PCM) transmission of speech and also to speech coding such as parametric coding using vocoders.
In a vocoder the parameters can themselves be viewed as sampled quantized analogue waveforms which require only moderate transmission accuracy to enable reproduction of the original speech signal, hence the small numbers of bits required compared with conventional PCM. However it will be realised that errors in some of the bits, for example the most significant bits (MSB) will produce much greater subjective effects than errors in bits of lower significance. Conventional error correction codes for data transmission have equal error correcting power for any data bit, which may be a wasteful use of resources when the errors occur in bits of low significance. A way of improving matters in this respect is discussed in an article by G. Robert Redinbo entitled "The Optimum Mean-Square Decoding of General Block Codes" in Information and Control, Vol. 31 pages 341-363 (1976) which discusses a method as defined in the first paragraph. However, the result of the known decoding method is still liable to result in errors and it is an object of the present invention to enable these errors to be reduced.
According to the invention a method as defined in the first paragraph is characterised in that said estimate is calculated taking into account a prevailing condition. Said prevailing condition may be, for example, the current bit error rate in the received code words, the current probabilities of each individual bit of the received code words being in error, and/or recent history, i.e. the previously generated value v.sub.j (t-1). It has been found that calculating said conditional probability in this way enables the errors resulting from the known method to be reduced.